1. Field of the Invention
The present invention relates to a proton-exchange method of forming optical waveguides and, more particularly, to such a method which utilizes sulfuric acid as the proton donor source.
2. Description of the Prior Art
The ability to form light-guiding structures in optically transparent substrates is considered to be a critical requirement for most optically-based communication systems, where lithium niobate or lithium tantalate is often utilized as the optically transparent substrate material of choice. One conventional method of forming such waveguides is to diffuse metal (titanium, for example) into the surface of the substrate. The in-diffused metal will increase the index of refraction of the substrate in the affected region such that any light signal propagating through the substrate will be forced to travel through the waveguiding region. U.S. Pat. No. 4,284,663 issued to J. R. Carruthers on Aug. 18, 1981 discloses the basic principles of this method.
An alternative waveguide forming technique is often referred to as the proton-exchange method. Here, the substrate is immersed in an acid bath causing the hydrogen ions present in the acid to exchange locations with the lithium atoms in the substrate material. This exchange of protons and lithium atoms will result in changing the refractive index of the substrate material. An article entitled "Proton exchange for high-index waveguides in LiNbO.sub.3 " by J. L. Jackel et al. appearing in Applied Physics Letters, Vol. 41, No. 7, October 1982, at p. 607-8 reports on one embodiment of this technique which utilizes benzoic acid as the proton source of choice. As described in the reference, benzoic acid was chosen for both its low proton donor strength and its ability to effect as much as a 50% replacement of the lithium in a surface layer several microns thick without inducing major structural damage to the substrate. Palmitic acid has also been successfully used as an exchange medium, as discussed in the article entitled "Independent Control of Index and Profiles in Proton-Exchanged Lithium Niobate Guides" by M. De Micheli et al. appearing in Optics Letters, Vol. 8, No. 2, 1983, at p. 114-5. Indeed, both acids have been found to yield a change in refractive index of .DELTA.n.sub.e =0.12. U.S. Pat. No. 4,547,262 issued to W. Spillman, Jr. et al. on Oct. 15, 1985 discusses a method of forming thin film waveguide structures using this conventional benzoic acid proton exchange method with lithium tantalate substrates (lithium niobate substrates being reported by Spillman, Jr. et al. as suffering from instability as related to proton mobility). Spillman, Jr. et al. also provides a discussion on the need to perform an anneal step after the exchange process in order to drive the protons sufficiently far into the substrate material so as to form light-guiding channels of a useful depth.
Although effective guiding structures may be formed using a benzoic acid source in the proton-exchange method, some problems exist with utilizing this acid in a manufacturing environment. Firstly, since benzoic acid is a rather weak acid, the proton exchange must be accomplished at an elevated temperature (over 200.degree. C.) to achieve the desired change in the index of refraction. Since benzoic acid begins to sublime at approximately 120.degree. C., it is difficult to control the exact benzoic composition during prolonged use (such as on a manufacturing line), at the required proton-exchange temperature of over 200.degree. C. Thus it becomes difficult, if not impossible, to form waveguides of reproducible quality on the substrates as they are processed through the line. Further, the breakdown and eventual vaporization of the benzoic acid makes it highly dangerous and an unlikely candidate for use in a manufacturing facility where a large number of workers may be exposed to the vapors.
Thus, a need remains in the prior art for an alternative method of forming waveguides using the proton-exchange technique which is compatible with mass-production facilities.